Aetna Health Inc. defeated a request to a court to allow multiple groups of insureds and out-of-network providers to proceed as a class on their legal challenge over the reimbursement rates Aetna used to pay medical claims, which were allegedly flawed and illegal.

Class certification isn’t appropriate because of the varied nature of terms in the dozens of Aetna plans at issue in the case that would require a plan-by-plan and sometimes even a claim-by-claim analysis, Judge Katharine S. Hayden...